LAWS OF CHEMICAL COMBINATION. 45 



The above diagram shows the simple relation which exists between 

 combining volumes, and atomic and molecular weights ; that such a 

 relation exists is not surprising, if we remember the law of Avogadro, 

 which has been before stated, and which says that all gases under 

 equal conditions contain the same number of molecules. 



The weighing of equal volumes of gases consequently is identical 

 with the weighing of equal numbers of molecules. The molecular 

 weight of a substance therefore can be found by weighing this sub- 

 stance in the gaseous state and comparing with it the weight of an 

 equal volume of another gas, the molecular weight of which is known. 

 The gas usually adopted for this comparison is hydrogen. 



If, for instance, we weigh equal volumes of hydrogen, chlorine, 

 oxygen, hydrochloric acid gas, and steam, we find weights in the 

 proportion of 2, 70.8, 32, 36.4, and 18. These numbers express 

 also the molecular weights of these substances ; moreover, they show 

 that atomic and molecular weights of elements are not identical, but 

 that the latter weight is twice that of the atomic weight, or that the 

 v ./molecules of elements consist of two atoms. 1 



One litre of hydrogen at the freezing-point of water and under the ordinary 

 pressure of 15 pounds to the square inch, weighs 0.0896 gramme. This weight 

 of one litre of hydrogen is taken as the unit or standard of comparison for 

 gases, and is called one crith. A litre of oxygen weighs 16 criths, one of 

 chlorine 35.4 criths, one of steam 9 criths, etc. 



Theory (Law) of equivalents. Valence, or Quanti valence. 

 /When one element replaces another element in a compound, the! 

 I quantities of the two elements are said to be equivalent to each other/ 

 and according to the law of equivalents the replacement of elements 

 one by another takes place always in definite proportions. Formerly 

 it was believed that the atoms of all elements were equivalent one 

 with another ; accordingly, atomic weights were frequently designated 

 as equivalent weights. 



This view, however, is not correct, as it is found that one atom of 

 one element frequently displaces two or more atoms of another 

 element. This fact, as well as other considerations, has led to the 

 assumption of the quantivalence of atoms. This property will be 

 understood best by selecting for consideration a few compounds of 

 different elements with hydrogen. 



I. II. III. IV. 



HC1 H 2 



HBr H 2 S H 3 As H,Si 



III H 2 Se H 3 P 



1 A few exceptions to this general rule will be mentioned in the proper places. 



